1. Field of the Invention
The present invention relates to a radio selective calling receiver and, more particularly, to a radio selective calling receiver having an automatic time correction function capable of automatically correcting the time indicated by an internal timepiece using time information contained in the received signal format.
2. Description of the Prior Art
Some conventional radio selective calling receivers having a timepiece function have a function of automatically correcting the time indicated by an internal timepiece using time information (information representing the time generated from a base station) in the received signal format.
Japanese Unexamined Patent Publication No. 4-230890 discloses a radio selective calling receiver incorporating two timepieces for counting two different times with a time difference and having a function of correcting one of the timepieces on the basis of any deviation from the received reference time information. As shown in FIG. 1, this radio selective calling receiver determines received reference time information first (step (to be abbreviated as ST hereinafter) 191). If the time data is normal (the data has neither an error nor a value outside the range of time information), the first timepiece is corrected using the received reference time information (ST192). Next, information of "second" unit or less of the second timepiece is corrected on the basis of the received reference time information (ST194). In addition, information of "minute" unit or more of the second timepiece is corrected on the basis of the deviation between the reference time information and the first time (value before correction in ST192). In ST195 and ST196, the time counting operations of the first and second timepieces are continued, respectively. The contents of a selected timepiece (ST197) are displayed/output (ST198).
In the above-described prior art, in determining the received time information (reference time information), it is detected only whether the data has a bit error or a value outside the range of time information. For this reason, when the received reference time information has no bit error but a value that has changed within the range (replaced with another code due to a bit error), the time of the internal timepiece is erroneously corrected.
In the BCH (31 or 21) code which is a block error correction code often used for radio selective calling, the intercode distance (Hamming distance) is five. A bit error of two or less bits can be detected and corrected, although an error of three or more bits may be replaced with another code within the range of a 2-bit error (replacement with another code due to a bit error will be simply referred to as a "change" hereinafter).
In the conventional signal scheme, a binary signal is modulated and transmitted. However, along with a recent increase in transfer rate of radio selective calling, as a means of increasing the transfer rate, a technique of modulating a quarternary signal and transmitting the modulated signal, as shown in FIG. 2, is becoming popular. The transmission signal shown in FIG. 2 is a quarternary signal (00, 01, 11, 10). This value is compared with a predetermined frame synchronization signal pattern, thereby detecting synchronization. In the example shown in FIG. 2, the frame synchronization signal pattern is "10, 00, 10, 10, 00, 10, 00, 00, 10, 10, 00, 00, 10, 10, 10". The same pattern as this frame synchronization signal pattern is detected from the transmission signal, and a synchronization signal detection signal is sent at the end timing of the pattern.
In this case, the quarternary signal is divided into an MSB (Most Significant Bit) and an LSB (Least Significant Bit). When the value of the MSB changes as "1, 0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 1, 1" and the value of the LSB does not change from "0", a synchronization signal detection signal can be detected. This state is shown in FIG. 2. The value of the MSB changes as described above, and the value of the LSB remains "0".
FIG. 3 is a block diagram showing a specific arrangement for detecting the frame synchronization signal pattern. As shown in FIG. 3, the MSB of the transmission signal is input to a 15-bit shift register 22, and at the same time, the LSB is input to a 15-bit shift register 23. A frame synchronization signal pattern generation unit 21 sends a frame synchronization signal pattern whose value changes as described above. A pattern matching circuit 24 compares the sent frame synchronization signal pattern with outputs from the 15-bit shift registers 22 and 23.
When the comparison result from the pattern matching circuit 24 reveals coincidence, detection of a frame synchronization signal pattern is determined, so a synchronization detection signal S106 is sent. To break up the quarternary signal into the MSB and LSB, level detection may be performed by an A/D converter.
In transmission of the quaternary signal, a preamble signal used to detect the intermediate level of the signal or detect bit synchronization, or a frame synchronization signal used to detect frame synchronization has only the highest and lowest levels of the quarternary signal, so it is substantially equivalent to a binary signal. For this reason, when the sensitivity of the receiver is not so high after establishment of frame synchronization, four-bit reference time information next to the frame synchronization signal is likely to be received with an error or change although the frame synchronization signal pattern can be detected.
In the above-described prior art, when the receiver user wants to intentionally set a time shifted from the reference time in the internal timepiece, the receiver must incorporate a plurality of timepiece functions. The plurality of timepiece functions increase the scale of the internal circuit, or an increase in CPU processing results in an increase in entire current consumption of the receiver. This is a serious disadvantage for the radio selective calling receiver driven by a battery.
In addition, the user cannot arbitrarily enable/disable time correction of the internal timepiece on the basis of the received reference time information.